Cross drainage work.pptx

cross drainage works

1. CROSS DRAINAGE WORKS

2. Introduction : A cross drainage work is a structure constructed for carrying a canal across a natural drain or river intercepting the canal. In order to minimize the number of cross drainage works , canals are generally aligned on a watershed so that they intercept a minimum number of natural drains. However , a canal taking off from a river requires a certain distance before it can mount the watershed. Once the canal is taken to the watershed , no cross drainage works are normally required. However , when the canal is aligned as a contour canal , a number of cross drainage works are necessary.

3. A cross drainage work is an expensive structure and should be avoided as far as possible. The number of cross drainage works can be reduced by Changing the alignment of the canal. Diverting small drainage into large drainages. Canal crossing natural drains

4. Types of cross drainage works : Depending upon the relative bed levels and discharge , the cross drainage works may be of the following types. Cross drainage works carrying canal over the natural drain : Aqueduct Syphon aqueduct Cross drainage works carrying the natural drain over the canal : Super passage Canal syphon c) Cross drainage works admitting the drain water into the canal : Level crossing Inlets and outlets

5. CROSS DRAINAGE WORKS CARRYING CANAL OVER THE NATURAL DRAIN : In this type of C.D. work, the canal is carried over the natural drain. The advantage of such arrangement is that the canal running perennially is above the ground level and is open to inspection. Also , the damage due to floods I rare. The structures that under this type are : Aqueduct Syphon aqueduct Aqueduct : An aqueduct is a structure in which the canal flows over the drain and the bed of the canal is well above the H.F.L. of the drain. The canal water is taken across the drain in a trough supported on piers.

6. Aqueduct and Syphon Aqueduct

7. Syphon Aqueduct

8. 2. Syphon Aqueduct : A syphon aqueduct is also a structure in which the canal flows over the drain but the H.F.L. of the drain is higher than the canal bed. The drain water flows under syphonic action through the aqueduct barrels. When sufficient level difference is not available between the canal bed and the H.F.L. of the drain to pass the drainage water, the bed of the drainage may be depressed of the drain to pass the drainage water, the bed of the drainage may be depressed below its normal bed level. The drain is provided with an impervious floor at the crossing and thus a barrel is formed between the piers to pass the drainage water under pressure. These barrels actually form an inverted syphon and not syphon .

9. SYPHON AQUEDUCT- NARMADA MAIN CANAL ON RIVER MEN

10. b) CROSS DRAINAGE WORKS CARRYING THE NATURAL DRAIN OVER THE CANAL : In this type of C.D. works the natural drain is carried over the canal. The advantage of this type is that the C.D. works themselves are less liable to damage than the earth work of canal. The structures that fall under this type are : Super passage Canal syphon

11. Super passage : A super passage is just like an aqueduct, except that in this case the drain is over the canal. The F.S.L. of the canal is lower than the under side of the drain trough. Thus , the canal water runs under gravity. The drain water is taken in a trough supported over the piers constructed on the bed of canal. Canal syphon : A canal syphon is constructed where the F.S.L. of the canal is higher than the bed level of drain trough. The canal water flows under syphonic action under the drain trough. The canal bed is lowered and a ramp is provided at the exit so that the trouble of sitting is minimized.

12. Super passage and canal syphon

13. SUPER PASSAGE

14. c) CROSS DRAINAGE WORKS ADMITTING THE DRAIN WATER INTO THE CANAL : In this type of C.D. works the canal water and the drain water are allowed to intermingle with each other. The only advantage of this type of work is its low initial cost. Such type of works have the following disadvantages : i . Regulation of such a work is difficult. ii. The faulty regulation of gates may damage the canal. iii. The canal has to be designed to carry the increased flood discharge of the drain. iv. There is additional expenditure of silt clearance.

15. Following are the structures under this type : Level crossing Inlet and outlets 1. LEVEL CROSSING : A level crossing is provided when the canal and the drain are practically at the same level. In a level crossing the drain water is admitted into the canal at one bank and is taken out at the opposite bank. A level crossing is consists of , A crest with its top at the F.S.L. of the canal across the drain at its upstream junction with the canal. A regulator with quick falling shutters across the drain at its downstream junction with the canal. A cross regulator across the canal at its downstream junction with the drain.

16. Level crossing

17. 2. INLET AND OUTLETS : An inlet is an open cut or a pipe which is provided in a canal bank to admit water into the canal. A canal inlet is provided when cross drainage flow is small and its water may be absorbed into the canal without causing appreciable rise. However , if the canal is small , an outlets may be constructed to pass out the additional discharge which has entered the canal. Such arrangement is called inlet and outlet. It is not necessary that the number of inlets and outlets should be the same. There may be one outlet for two or three inlets.

18. Inlet

19. SELECTION OF SUITABLE TYPE OF CD WORKS : The following factors should be considered while selecting the most suitable type of the cross drainage work. Relative levels and discharge : The relative levels and discharge of the canal and of the drain largely affect the type of cross drainage work required. The main outlines are : If the canal bed level is sufficiently above the H.F.L. of the drain, an aqueduct is selected. If the H.F.L. of the drain is higher than the canal bed, a syphon aqueduct is provided. If the F.S.L. of the canal is lower than the underside of the drain trough, a super passage is provided. If the F.S.L. of the canal is slightly above the bed level of the drain and the canal is of small size , a canal syphon is provided.

20. BOX CULVERT

21. 2. Type of flow : As far as possible , the structure having and open channel flow should be preferred to the structure having a pipe flow. Therefore , an aqueduct should be preferred to a syphon aqueduct and a super passage should be preferred to a canal syphon . Size of drain : When the drain is of small size, a syphon aqueduct will be preferred to an aqueduct as the construction of aqueduct involves high banks and long approaches . However, if the drain is of large size, an aqueduct is preferred. 4. Materials of construction : Sufficient quantity of construction materials like sand, aggregate, gravel, boulders, earth, etc. should be available near the site for the type of C.D. work selected. Soil in sufficient quantity should be available for constructing the canal banks if the C.D. structure requires long and high canal banks.

22. Foundation : The type of C.D. work should be selected depending upon the foundation available at the site work. Cost of earthwork : The of C.D. work which does not involve a large quantity of earthwork of the canal should be preferred. Overall cost : The overall cost of the C.D. work including the cost of construction , canal banks , maintenance , etc. should be a minimum. Provision of road bridge : A aqueduct is better than a super passage because in the former, a road bridge can easily be provided along with the canal trough at a small additional cost, whereas in the latter , a separate road bridge is required. Subsoil water table : If the subsoil water table is high, the types of cross-drainage which requires excavation should be avoided, as it would involve dewatering problems.

23. CLASSIFICATION OF AQUEDUCTS AND SYPHON AQUEDUCTS : Depending upon the nature of the sides, aqueducts or syphon aqueducts are classified as under : TYPE I : In this type the slides of the aqueduct (or syphon aqueduct ) are in earthen banks with complete earthen slopes. The original canal section is retained and no fluming is done. TYPE II : In this type also the canal continues in its earthen section over the drain , but the outer slopes of the earthen banks are replaced by retaining walls , thereby reducing the length of the barrels by that extent.

24. TYPE III : In this type the earthen banks are not provided through the aqueduct , fluming is done , and canal water is carried in a concrete trough. The sides of the trough are connected on either side of the work to earthen banks of the canal through wing walls. Thus in this case the length of drain barrels is reduced. Type III Aqueduct

25. SELECTION OF SITE OF A CD WORK : The following points should be considered while selecting the site of a CD work. The stream at the site should be stable and should have stable banks. At the site, the drain should cross the canal alignment at right angles. The site should be such that long and high approaches of the canal are not required. A firm and strong sub-stratum should be available below the bed of the drain for economical design and construction of foundations. The length and height of the marginal banks and guide banks should be small. The water table at the site should be low. In case of an aqueduct, sufficient headway should be available between the H.F.L. of the drain and underside of the canal trough. The possibility of diverting one drain into the another upstream of the canal crossing should also be considered.

26. DETERMINATION OF MAXIMUM FLOOD DISCHARGE : The estimation of peak flood discharge is useful in designing the structures required for the disposal of the surplus water or excess water. The estimation of the peak flood discharge can be made by the following methods : By physical indication of past floods By empirical flood formulae By envelope curves By rational method By flood frequency studies Unit hydrograph method

27. By physical indication of past flood : The following procedure is used to estimate the flood discharge. The flood marks along the reach of the river are connected by levelling to determine the water surface levels. The slope ‘s’ of the water surface is calculated from the difference of high flood levels over a known distance. ii. The profile of the river is plotted & the cross-sectional area of the river (A) is computed up to the high flood level mark. The value of hydraulic mean depth (R) id the computed. iii. Manning’s coefficient (N) is assumed. The discharge (Q) is computed as under. Where,

28. 1. By Empirical flood formula : Dicken’s formula : Where , Q = Maximum flood discharge ( cumecs ) A = Area of the catchment (km²) C = Dicken’s coefficient The value of C for different regions are given below: Region Value of C North India 11.4 Central India 13.9 – 19.5 Western India 22.2 - 25

29. 2. Ryve’s formula : Where , Q = Maximum flood discharge ( cumecs ) A = Area of the catchment (km²) C = Ryve’s coefficient The value of C for different locations are given in table: Location of the catchment Value of C 1. Area within 24 km from the coast 6.75 2. Area between 24 km to 161 km from the coast 8.45 3. Limited areas near hills 10.0

30. 3. Inglis formula : Where , Q = Maximum flood discharge ( cumecs ) A = Area of the catchment (km²) 4. All Nawaz Jung Bahadur’s formula : Where , Q = Maximum flood discharge ( cumecs ) A = Area of the catchment (km²) C = Coefficient , the value of c varies from 48 to 60. The maximum value of C is 85.

31. 5. Fanning’ s formula : For American catchments, Where, average value of C may be taken equal to 2.54.

32. If plotting is done on a log – log paper then the envelop curve is usually a straight line. For Indian rivers envelope curves from observed data of floods have been developed by Kanwar Sain & Karpov . 2. By Envelope Curves : In this method, areas having similar topographical features & climatic conditions are grouped together & the available data regarding discharges are complied along with their catchment areas. The maximum flood discharges are then plotted against the areas of the drainage basins & a curve is drawn to cover the highest plotted, which is known as envelope curve.

33. 3. Rational Method : Where , Q = Maximum flood discharge ( cumecs ) A = Area of the catchment (km²) = Critical intensity of rainfall (cm/ hr) C = coefficient which depends upon the characteristics of the catchment Where , P = Maximum precipitation in (cm) = Storm period (hours) = time of concentration (hours)

34. 4. By flood frequency studies : Flood frequency (F) : Flood frequency means the number of times a flood of given magnitude will be equalled or exceeded in any one year. It is denoted by F. A 10% frequency means that the flood has 10 out of 100 chances of being equalled or exceed. Recurrance interval (T) : Recurrance interval denotes the number of years in which a flood of given magnitude can be expected once. It is denoted by T.

35. Allen – Hazen method : Weibull method: Gumbel method : i . California method:

36. DETERMINATION OF WATERWAY OF THE DRAIN : In plains, the drainage are generally in alluvium & waterway usually providing works without rigid floor is about 60 to 80% of perimeter given by Lacey’s formula. Where , = Wetted perimeter in m C = A coefficient varying from 4.5 to 6.3 according to local conditions, the usual value is 4.8 for regime channels. Q = discharge in m³/s

37. FLUMING OF CANAL OR CONTRACTION OF CANAL WATERWAY : When the canal waterway is contracted over the crossing , it is known as fluming of canal. The fluming or the contraction of the canal waterway should be done in such a way that the velocity in the trough is not more than 3 m/s and the flow remain subcritical to avoid the possibility of formation of hydraulic jump in the trough . The approach transition wings should not be steeper than 30° and the departure transition should not be steeper than 22(1/2)° . The transition consists of curved and flared wing walls so that there is minimum loss of head and the flow is streamlined.    

38. The transition can be designed under two different conditions: Design of transition when the water depth remains constant. Design of transition when water depth varies. Design of transition when the water depth remains constant : Under this condition , the following two methods may be adopted for the design of transition. Mitra’s hyperbolic transition: Chaturvedi’s semi-cubical parabolic transition:

39. Mitra’s hyperbolic transition : A.C Mitra (1940) proposed a hyperbolic transition based on the following two assumptions. i . The depth of flow remains constant. ii . The rate of change of velocity along the length of transition remains constant. Let, Bo = original or normal width of canal section Bf = flumed width of canal section Bx = width at any distance x from the flumed section Let , Lf = total length of transition The transition is designed on the basis that the rate of change of Velocity per unit length of the transition is constant throughout the transition length.

40. Contraction of canal waterway

41. = Let , D = water depth Q = discharge From continuity equation, V f × B f × D = V x × B x × D = V o × B o × D = Q V f × B f = V n × B x = V o × B o = Q/D = constant =K Hence, ……………………( i ) V f T hus,

42. V x V o Substituting this value in eq . ( i ), we get = …………………………….(ii) This is Mitra’s hyperbolic transition for bed width at any distance x from the section .

43. 1 – [ ] Chaturvedi’s semi-cubical parabolic transition : On the basis of his experiments Prof. R. S. Chaturvedi proposed the following equation of transition for constant water depth. Choosing various convenient values of B x , the corresponding distance x can be computed from the above equation.

44. Design of transition when water depth varies : Hind method : When the water depth in the trough and the normal section of the canal vary, Hind’s method given below may be used. The following figure shows the expansion and contraction transitions . The contraction starts at section 1-1 and ends at section 2-2 . The canal section remains flumed from section 2-2 to 3-3 . The expansion transition starts from section 3-3 and ends at Section 4-4 .let D and V with appropriate suffixes represent depth and velocity at different sections.

45. DESIGN PROCEDURE : STEP-1: Let the bed level and cross-section of the canal at section 4-4 be completely known. water surface elevation at section 4-4 = Bed level at section 4-4 + D 4 Therefore , T.E.L. at section 4-4 = water surface elevation at section 4-4 + V 2 4 / 2g STEP-2: Between section 3-3 and 4-4 there is energy loss due to expansion and friction. Energy loss due to expansion = 0.3(V 3 2 - V 4 2 / 2g) The energy loss due to friction is generally small and may be neglected. Since, trough dimensions at section 3-3 are known, V 3 is known.

46. STEP-3 : T.E.L. at section 3-3 = T.E.L. at section 4-4 + 0.3(V 3 2 - V 4 2 / 2g) Water surface elevation at 3-3 = T.E.L. at section 3-3 – (V 3 2 / 2g) Bed level at 3-3 = Water surface elevation at 3-3 – D 3 STEP-4 : The channel section between 3 and 2 remains constant. The only loss of head in the trough is due to friction (H L ), which can be computed by Manning’s formula.

47. T.E.L. at section 2-2 =T.E.L. at 3-3 + H L water surface elevation at 2-2 = T.E.L. at 2-2 - (V 2 2 / 2g) Bed level at section 2-2 = water surface elevation at 2-2 - D 2 The depth and velocity in the trough are constant throughout and hence T.E.L. , water surface line and bed line are parallel to each other from section 2-2 to 3-3.

48. STEP-5 : Energy loss due to contraction between section 1-1 and 2-2 may be taken equal to, = 0.2(V 2 2 – V 1 2 )/2g The friction loss may be neglected. T.E.L. at section 1-1 = T.E.L. at 2-2 + 0.2 (V 2 2 – V 1 2 )/2g Water surface elevation at 1-1 = T.E.L. at 1-1 – ( V 1 2 /2g ) Bed level at section 1-1 = Water surface elevation at 1-1-D 1 STEP-6 : The bed level , water surface level and total energy level at all for sections are known. The T.E.L. is drawn assuming it to be a straight line between the adjacent sections.

49. The bed line is also drawn straight if the fall in level between the adjacent sections is small, and the corners can be rounded off. In case the drop in the bed is more, the sections are joined with a smooth reverse curve tangential to the bed. STEP-7 : The drop in the water surface levels between any two adjacent sections will be equal to , Drop in the energy line between the two sections. Increased velocity head for contraction (or decreased velocity head for expansion) This drop is achieved by two smooth parabolic curves , first convex and s econd concave upwards, meeting the water surface tangentially, as shown in Fig.

50. Water surface profile for contraction transition

51. Let , 2x 1 = L f = length of fluming 2y 1 = total difference in water levels between section 1-1 and 2-2. The distance of the middle point of transition will be x 1 and drop in water surface be y 1. The equation of parabola , with origin at water surface of section 1-1, is given by y = cx 2 at x = x 1 , y = y 1 Therefore, c = y/x 2 = y 1 / x 1 2 Hence the equation becomes , Y = (y 1 / x 1 2 ) x 2

52. This is the equation of first parabola curve , which can now be plotted. similarly , the second parabola can be plotted by taking the origin at section 2. Similarly , the expansion transition water surface between section 3-3 and 4-4 can be plotted . There will be rise in the water surface. STEP-8 : After plotting the water surface profile the velocity head h a can be found by measuring the vertical distance between the T.E.L. and the water surface line at any point . The velocity head can be converted into equivalent velocity by the relation. V = Thus the velocity at each point can be known.

53. STEP-9 : The cross sectional area A required to pass the discharge Q is given by A = Q/V As the bed line is also drawn before hand the depth D at every section is known. In a trapezoidal channel , of bed width B , depth D and side slopes n:1, the area is given by A = BD + n D 2 In flared wings , the side slopes are gradually brought to vertical from an initial slope and the side slope at any section can be interpolated in proportion to the length of transition undergone. Thus at any section A, D and n are known , the bed width B may be obtained by the above equation .

54. UPLIFT PRESSURE ON THE FLOOR OF THE BARREL OR CULVERT : The floor of the culverts or barrels is subjected to uplift pressure due to the following two cases : Uplift pressure due to seepage of water from the canal to the drain : due to difference of head between the canal and the drain, seepage flow will take place from the canal to the drain which causes uplift pressure on the floor of the culverts or barrels. The maximum uplift pressure due to seepage will occur when the canal is running full, but there is no water in the drain.

55. Uplift pressure due to subsoil water in the drain bed : This uplift pressure acts when the floor of the barrels of culverts is depressed below the bed of the drain. The maximum uplift pressure under the worst condition would occur when there is no water flowing in the drain and the water table has risen up to the drain bed. Bligh’s creep theory can be used to find the uplift pressure and to design the floor. As Bligh’s method, the flow starts from the point A, underside of the canal trough at its u/s end and then comes out at the point C under the center of the floor of the first culvert or barrel at its end. Point B is the point under the center of the floor of the first culvert. The seepage path from A to B and from B to C can be known.

56. Uplift pressure on barrel floor

57. The total creep length L will then be equal to sum of seepage path L 1 from A to B and seepage path L 2 from B to C . L = L 1 + L 2 If H = total seepage head = F.S.L. of canal – d/s bed level of drain The residual seepage head at point B is given by H r = x L 2 The residual seepage head ( H r ) may be used for the design of the entire floor of the barrels or culverts.

58. Methods of reducing uplift on the floor : The uplift pressure on the floor can be reduced by the following two methods. By extending the impervious floor of the canal trough on either side. The creep length is thus increased and the uplift pressure is reduced. By providing the drainage holes or relief holes in the floor of the aqueduct, the uplift pressure can be released. An inverted filter may be provided below the floor so that the subsoil particles are not lifted up. It would also be necessary to provide flap valves on the top of relief holes, so that seepage water from beneath may come up but the silt laden water may not enter the filter and choke it.

59. DESIGN OF BANK CONNECYIONS : Two types of wings are required to be constructed in aqueducts and syphon aqueducts. River wings or drain wings Canal wings 1. River wings or drain wings : River wings retain and protect the earthen slopes of the canal, guide the drain water entering and leaving the work, and join it to the guide banks. These wings should also be designed walls for maximum earth pressure likely to come on them with no water in the drain. These wings also provide vertical cutoff for the water seeping from the canal into the drain bed.

60. Canal wings : The canal wings are generally warped to change the canal section from trapezoidal to rectangular in the trough. The wings are extended to the end of splay. They should be designed as retaining walls, with no water in the canal. They should be taken deep enough to give safe creep length.

61. CANAL REGULATION WORKS

62. INTRODUCTION : The structures constructed on a canal to regulate the discharge, full supply level or velocity of flow are known as canal regulation works . The structures are necessary for efficient functioning of an irrigation canal system. The various regulation works are: Canal falls Cross regulator Distributary head regulator Canal escapes

63. CANAL FALLS : A canal fall is an irrigation structure constructed across a canal to lower down its water level and destory the surplus energy liberated from the falling water which may otherwise scour the bed and banks of the canal. Necessity and Location of falls : The canal falls are required when the slope of the ground along the canal alignment is steeper than the bed slope of the canal. The following factors should be considered while selecting the location of the fall :

64. For the canal which does not irrigate the area directly, the fall should be located from the consideration of economy of earthwork. As far as possible the canal should be kept in the balanced depth of cutting. 2. For a canal irrigating the area directly, a fall may be provided at a location where the F.S.L. of the canal outstrips the ground level but before the bed of the canal comes into filling. 3. The site for the fall in the case of distributaries from which direct irrigation is done, decided in such a way that command is not sacrificed in the process of lowering of the water level. After the fall, the F.S.L. of the canal may be below the ground level for ½ to ¾ kilometer. 4. The location of a fall may also be decided from the consideration of the possibility of combining with a cross regulator or a road bridge to effect economy and to have better regulation.

65. A relative economy of providing a large number of small falls or small number of large falls should also be worked out. The provision of small number of big falls results in unbalanced earthwork, but there is always some saving in the cost of the fall structure. Sometimes it may be necessary to provide fewer falls of large drops to enable hydropower generation at these falls.

66. TYPES OF FALLS : The falls were first constructed by British in India in the nineteenth century. The various types of falls are : Ogee fall Rapid fall Stepped fall Notch fall Vertical drop fall Glacis fall Sarda type fall

67. Ogee fall : The ogee fall was first constructed by Sir Proby Cantley on the Ganga canal. This type of fall has gradual convex and concave curves with an aim to provide a smooth transition and to reduce distrubance and impact. This preserved the energy without dissipating it. Due to this, the ogee fall has the following shortcomings : On account of heavy drawdown on the u/s side of the fall there was erosion of bed and banks of the canal. Due to smooth transition, the kinetic energy was fully preserved which caused erosion of bed and banks of the channel on the d/s side of the fall.

68. Ogee fall

69. Rapid fall : The rapid fall was evolved by R.F. Croften and first constructed on the Western Yamuna Canals. Such a fall consists of a glacis sloping at 1 vertical to 10 to 20 horizontal. The long glacis assured the formation of the hydraulic jump for the dissipation of energy. Morever , these falls were also used for timber traffic. However, the falls could not become popular because of their high cost of construction.

70. Rapid fall

71. RAPID FALL

72. 3. Stepped fall : The stepped falls were the modified forms of the rapid fall, in which the long glacis was replaced by a long, stepped floor. This type of fall also could not become popular because of its high cost construction. After the development of stepped falls, it was recognised that the dissipation of energy can be achieved through vertical impact of the falling jet of water on the floor. However, the earlier types of vertical falls were not well developed and gave trouble. As such these were superseded for some time by trapezoidal notch falls.

73. Stepped fall

74. STEPPED FALL

75. Notch fall : A trapezoidal notch fall consists of a number of trapezoidal notches in high breast wall, called notch pier, constructed across the channel. There is a smooth entrance to the notches. A flat, circular lip projecting down stream from each notch disperses the water. The notches of the fall were designed to maintain the normal depth of flow in the channel upstream of the flow at any two discharge values.

76. Trapezoidal notch fall

77. NOTCH FALL

78. Vertical drop fall : In the vertical drop fall a crest wall is constructed to create a vertical drop. The cistern is provided to dissipate the surplus energy of water leaving the crest. In the cistern a grid consisting of banks of timber placed a few centimeters apart was provided to intercept the falling nappe and thus dissipate its surplus energy. This fall did not become popular because of its getting clogged with floating debris. Morever , the decay of timber necessitated frequent replacement. The Sharda type fall developed on the Sharda canal project in U.P. is a recent type of vertical drop type fall which has proved quite successful.

79. Vertical drop fall

80. VERTICAL FALL

81. Glacis fall : The Glacis fall utilises hydraulic jump (or standing wave) for the dissipation of energy. There are three types of glacis fall : Straight glacis fall Parabolic glacis or Montague type fall Inglis fall i . Straight glacis : In Punjab, the flumed fall with straight glacis was developed. It utilises the formation of a hydraulic jump for the dissipation of energy. However, there was serious trouble with some of these falls.

82. One cause of trouble was that even after the formation of hydraulic jump, there was considerable surplus energy in water. The another cause of trouble was due to too rapid expansion after fluming eddies were developed which caused deep scour. Straight glacis fall

83. GLACIS FALL

84. Montague fall : This type is a modified form of the straight glacis fall. In this, a parabolic glacis, known as the Montague profile is provided. This shape gives the maximum horizontal acceleration to the jet of water in a given length of glacis. This type fall provides a solution to the problem of residual energy after the formation of the hydraulic jump on a straight glacis. It is not possible to dissipate the entire energy and considerable surplus energy is still left even after formation of the hydraulic jump.

85. Montague fall

86. Inglis fall : It is also a modified form of the straight glacis fall. In this type of fall a baffle wall of certain height is provided at some distance d/s of the toe of the straight glacis. The baffle wall ensures the formation of the hydraulic jump on the baffle platform and effective dissipation of energy. Inglis fall

87. Sarda type fall : The Sarda type fall is a vertical drop fall in which there is araised crest and there is a vertical impact of the falling jet. This type of fall was first introduced to replace the notch fall on the Sarda canal system in U.P. owing to its economy and simplicity. The maximum height of drop was 1.8 m. The depth of cutting was therefore kept small to avoid excavating the pure sand stratum and increasing seepage losses. In the earlier design of these falls, no depressed cistern was provided and the d/s winds were not flared.

88. Sarda type fall

89. CLASSIFICATION OF FALLS : With reference to the normal condition, the falls may be divided into four classes : Class-I : Falls designed to maintain the normal depth-discharge ratio In this class, there is neither drawdown nor heading up of water as the flow in the channel approaches the fall. Hence, the different discharges, more or less normal uniform flow is maintained in the channel upatream . The falls under this class are Trapezoidal notch fall Rectangular notch fall or Low crested weir

90. Class-II : Falls designed to maintain a fixed water level in the channel upstream of the fall In this class, those falls are included which ,maintain a nearly fixed water level in the upstream of the fall. This type of falls are necessary under the following circumstances. When a subsidiary channel takes off some distance upstream of the fall. When a hydropower station is combined with the fall. The falls under this class are Siphon falls (siphon spillways) High crested weir falls

91. Class-III : Falls designed to admit the variation of the water level in the channel upstream The falls under this class are designed to admit the variations of water level on the upstream of the fall according to the requirements. They also serve as regulators. The regulation is done with the help of sluice gates horizontal stop logs vertical needles The fall of this class is rectangular notch fall.

92. Class-IV : Miscellaneous falls The principal types of falls in this class are Cylindrical falls or well falls Chute (or rapid) falls Inclined falls CHUTE FALL

93. ALIGNMENT OF THE OFF-TAKING CHANNEL : When a distributing channel takes off from the parent channel, their off-take alignment should be carefully designed. The alignment should be such that the off taking channel is able to draw its supply without any undesirable effect. The following types of alignment are commonly adopted in practice. The best alignment of the off taking channel is when it makes zero angle with the parent channel initially and then separates out gradually along a transition curve.(fig.-a) If transition curve should be properly designed to avoid accumulation of silt in the form of the silt jetty and to ensure equitable distribution of silt.

94. Off-take alignment

95. If the transition curves are not provided, the alignment shown in fig.-b may be adopted. In this case, the off taking channel and the parent channel on downstream make an angle with the parent channel on upstream of the offtake point. If it is essential to keep the parent channel straight, the edge of the channel rather than the center line should be considered while designing the angle of offtake . An angle of 60° to 80° is generally quite suitable.(fig.-c) Fig.-d shows an unbalanced offtake , which should be avoided as far as possible. In that circumstances, the section should not be narrowed down equally on both sides. An unbalanced offtake results in the formation of a jetty.

96. CROSS REGULATOR : A cross regulator is provided on the parent channel at the downstream of the offtaking canal to head up the water level and to enable the offtaking channel to draw the required supply. Functions of cross regulator : Cross regulator enable effective regulation of the entire canal system. During the periods of low discharge in the parent channel, the cross regulator raises water level on the u/s so that the offtaking channel can take its full supply. It helps in closing the supply to the d/s of the parent channel, for the purpose of repairs. There is usually a bridge on the cross-regulator, which provides a means of communication.

97. 5. It can be easily combined with a canal fall, in which case, it helps to control the water surface slope. It helps to absorb fluctuations in the various sections to the canal system, and thus prevents breaches in the tail reaches. It can be used to control the drawdown when the subsoil water levels are high to ensure safety of canal lining. In conjuction with escapes they help water to escape from the channels.

98. CROSS REGULATOR

99. DISTRIBUTARY HEAD REGULATOR : A distributary head regulator is provided at the head of the offtaking canal to control the supplies entering the offtaking canal or distributary . Distributary head regulator

100. Functions of distributary head regulator : They regulate or control the supply of water to the offtaking channel from the parent channel. 2. They control the entry of silt in the offtaking channel. 3. They serve as meter for measuring the discharge entering into the offtaking canal. They help in shutting off the supplies when not needed in the offtaking channel or when the offtaking channel is required to be closed for repairs.

101. DISTRIBUTARY HEAD REGULATOR

102. CANAL ESCAPES : A canal escape is a structure constructed on an irrigation canal for the disposal of surplus water from the canal . It is a sort of safety valve. Necessity of surplus water escape : A mistake or difficulty in regulation at the head of the channel. Heavy rain fall in the upper reaches of the channel when the channel is already running full. Sudden closures of outlets by cultivators due to a decrease in demand of water. Sudden closures of any offtaking channel due to breach.

103. TYPES OF ESCAPES : Classification based on the purpose : Surplus water escape Canal scouring escape Tail escape Classification based on the structural design : Regulator type escape or sluice type escapes Weir type escapes

104. Surplus water escape : A surplus water escape is a structure constructed on an irrigation channel to dispose of surplus water from the channel. It is known as canal surplus escape. Surplus water escape

105. These escape are provided in the banks of the channel at intervals depending on the importance of the channel and the vicinity of a suitable natural drain or river for disposal of surplus water. These channel leading surplus water from escape to natural drain is called escape channel. The capacity of the escape channel may be 1/3 to ½ of the capacity of the channel. 2. Canal scouring escapes : The canal scouring escape is constructed in the bank of the canal for the purpose of scouring off excess silt from time to time . These escapes are usually provided only in head reaches of main canals. The discharge capacity of the canal scouring escape should be about 1/2 to 2/3 of the capacity of the main canal at the head.

106. Tail escape : An escape is provided across the channel at its tail end to maintain the required F.S.L. at the tail end. Such an escape is called tail escape. Tail escape

107. SLUICE ESCAPE

108. SILT CONTROL DEVICES : In order to control the silt entry into the off taking canal the distributary head regulator is provided with a raised crest. Raised crest : If the crest level of the distributary head regulator is kept 0.3 to 0.6 m higher than the u/s bed level of the parent channel, the silt entry into the offtaking canal is considerably reduced. However, the raised crest is not enough by it self to control the silt because if there is turbulance at the head, the top water may carry , as much or even more silt as the bottom water.

109. Therefore to reduce the silt entry into the offtaking channel, following devices are used. 1. King’s vanes 2. Gibb’s groyne wall 3. Cantilever skimming platform 4. Curved wings 5. Curved wings with sediment vanes 6. Desilting basins 7. Vortex tube

110. King’s vanes : Kings vanes are vertical diaphragm walls which are parallel to one another and curved at a radius of 7.5 m or more. These are of low height and are provided in the parent channel at the head of offtaking channel such that they deflect the bottom silt laden water from the offtaking channel at an angle of 30° from the direction of flow & thus help to prevent the entry of silt into the offtaking canal. The vanes may be of 75 mm RCC or steel plates. The height of vanes is ¼ the deoth of water in the parent channel. The spacing of vanes is 1½ times their height. The bed of the parent chnnel covered by vanes & also for a distance of 15 to 30 m on the upstream of vanes is pitched at bed & sides so that it becomes smooth & the suspended sediments fall near the canal bed.

111. King’s vanes

112. Gibb’s groyne wall : Gibb’s groyne wall is an extension of the d/s abutment of the offtaking channel into the parent channel. The groyne wall extends up to the upstream abutment or atleast ¾ times the width of the offtake channel. The groyne wall is located so that it divides the total discharge (Q) of the parent channel in proportion of the discharge in offtaking channel (Q1) & that in the d/s parent chanel (Q – Q1) . Groyne wall should take in more discharge than required & the surplus should be escaped through a hole about 1 m dia. in the groyne wall near its bottom.

113. Gibb’s groyne wall

114. Cantilever skimming platform : A cantilever skimming platform consist of a slab or platform supported on piers constructed in the parent channel in the parent channel along the direction of the flow. It excludes the silt laden water in the bottom layers & hence the clear water in the top layers enters the offtaking channel. The minimum depth of the tunnel below the platform should be 0.6 m. Cantilever skimming platform

115. Curved wings : A curved wing is similar to Gibb’s groyne wall & consists of a vertical wall extending from the downstream abutment of the offtaking channel. The nose of the curved wing is inclined at 30° to 45° to direction of flow in the parent channel. The height of the curved wing is kept such that its top is atleast 0.3 m above the F.S.L. of the parent channel. Curved wings

116. 5. Curved wings with sediment vanes : If the bed slope of the offtaking channel is milder than that of the parent channel the curved wing alone is not effective in controlling the silt entry. In such a case, sediment vanes are provided in conjuction with the curved wing. The optimum radius of the vanes is about 12 m. However, it should be atleast 7.5 m to reduce eddy formation due to curvature of vanes, the vanes should be kept straight & should make an angle of about 27° with the direction of flow in the parent channel. The height of the sediment vanes is about 0.25 times the full supply depth of the parent channel. The spacing between the vanes is kept about 1.5 times their height.

117. Curved wings with sediment

118. Desilting basins : The desilting basins consists of low lying tanks along the canal banks, enclosed by high bunds. The canal water is allowed to pass over these tanks. Water, after dropping a major part of the silt in the tanks, flow back to the canal at the d/s end of the basin where an outlet is provided. Vortex tube : A vortex tube is a tube open at top & placed across the bottom of the channel. It is normally placed normal to the direction of flow or at any angle greater than 30° with the direction of flow.

119. As the channel water passes over the tube, a vortex motion is set up in the tube due to shearing action. This vortex prevents sediment deposition in the tube. Vortex tube

120. DESILTING BASIN